1. Field of the Invention
The present invention relates to an optical disc apparatus, a position control method and an optical pickup. The present invention provides a technique that can suitably be applied to an optical disc apparatus for recording information on a plurality of layers of an optical disc.
2. Description of the Related Art
Known popular optical disc apparatus are mostly designed to irradiate a light beam on an optical disc such as a compact disc (CD), a digital versatile disc (DVD) or a Blu-ray disc (registered trademark, to be referred to as “BD” hereinafter) and reproduce information by reading the reflected light beam.
Such known popular optical disc apparatus are also designed to record information on an optical disc by irradiating a light beam onto the optical disc and changing the local reflectance or the like of the optical disc.
As for such optical discs, it is known that the size of the light spot to be formed on the optical disc is defined approximately by λ/NA (λ: wavelength of light beam, NA: numerical aperture) and the resolution is proportional to the size of the light spot. For instance, with the BD system, about 25 GB of data can be recorded on an optical disc having a diameter of 120 mm.
Meanwhile, various pieces of information are recorded on optical discs. Information that can be recorded on optical discs includes audio contents, video contents and various computer data. Particularly, in recent years, the quantity of information to be recorded on an optical disc has rapidly grown because of an increasing demand for high definition images and high quality sounds and the increased number of sets of contents to be recorded on an optical disc so that optical discs are by turn required to have a large recording capacity.
We consider here that a plurality of recording layers is provided on one optical disk. In the case where an optical disk is formed by laminating layers (reflecting layer, recording layer, and the like) made of different materials as shown in FIG. 1A, which has been done in a conventional DVD system and BD system, manufacturing process becomes complicated, leading to increased cost.
Thus, there is proposed an optical disk apparatus that records information while laminating a plurality of layers of a recording mark (hereinafter, referred to as “mark layer”) in a uniform recording layer of an optical disk so as to increase a recording capacity (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2007-220206 (FIGS. 1, 4, and 5)).
As shown in FIG. 1B, an optical disk 100 that accepts the optical disk apparatus having the above configuration has a uniform recording layer 101. That is, there exists no mark for position identification in the recording layer. Therefore, in the optical disk 100, a reference layer 104 having a track and the like is provided.
In this case, the optical disk apparatus focuses a predetermined servo light beam LS onto a reference track (hereinafter, referred to as “track TE”) of the reference layer 104 to cause an information light beam LM having an optical axis XL coinciding with the optical axis of the servo light beam LS to focus onto a target position (hereinafter referred to as “target position PG”) in the recording layer 101.